Metallic nanocrystalline materials and metallic glasses may be considered to be special classes of materials known to exhibit relatively high hardness and strength characteristics. Due to their potential, they are considered to be candidates for structural applications. However, these classes of materials may exhibit limited fracture toughness associated with the rapid propagation of shear bands and/or cracks, which may be a concern for the technological utilization of these materials. While these materials may show adequate ductility by testing in compression, when testing in tension these materials may show elongations close to zero and in the brittle regime. The inherent inability of these classes of material to be able to deform in tension at room temperature may be a limited factor for some potential structural applications where intrinsic ductility is needed to avoid catastrophic failure.
In some cases, nanocrystalline materials may be understood as polycrystalline structures with a mean grain size below 500 nm including, in some cases, a mean grain size below 100 nm. Despite their relatively attractive properties (high hardness, yield stress and fracture strength), nanocrystalline materials may generally show a disappointing and relatively low tensile elongation and mat tend to fail in an extremely brittle manner. In fact, the decrease of ductility for decreasing grain sizes has been known for a long time as attested, for instance, by the empirical correlation between the work hardening exponent and the grain size proposed by others for cold rolled and conventionally recrystallized mild steels. As the grain size progressively decreases, the formation of dislocation pile-ups may become more difficult, limiting the capacity for strain hardening, which may lead to mechanical instability and cracking under loading.